The basic technique used to fabricate an integrated circuit is photolithography. According to the so-called planar process invented by the company Fairchild in 1959, a wafer of pure (to within 1 ppm) polycrystalline silicon approximately 0.7 mm in thickness and 200 mm in diameter, the surface of which has been polished in order to make it perfectly smooth, is covered with an etching layer consisting of a dielectric (for example SiO.sub.2) or of a metal (for example aluminium) and then the assembly is coated with a 1 to 2 .mu.m thick film of a photoresist (hereinafter called PR), i.e. a photosensitive resin composed of a polymer and a photosensitizer. Next, visible or advantageously ultraviolet light is projected through a mask, the opaque surfaces of which form the replica of the desired pattern, so that certain parts of the PR resin are exposed and others are not. After this exposure step, the PR resin is developed either, if it is a positive PR, in a basic solution which dissolves the exposed parts or, if it is a negative PR, in an organic solvent which dissolves the masked parts. In the recesses thus formed, the exposed region of the dielectric (SiO.sub.2, modified silica, silicon carbide) or metal (Al, Cu, Cr, W, etc.) layer is then removed by an acid solution or by plasma. After this so-called etching operation, the residue of PR resin must then be removed by a so-called PR stripping operation and, as shown in the diagram in the appended FIG. 1, a positive image or a negative image is finally obtained.
The fabrication of integrated circuits is an extremely elaborate industrial operation, of which the above description and the diagram in FIG. 1 give here merely a general idea. Its difficulty essentially resides in the small size of the objects which are handled, since the transistors currently fabricated have a gate width of 0.25 .mu.m. The fabrication must be carried out in chambers called clean rooms, from which dust is systematically removed; class I clean rooms contain less than 30 particles of size greater than 0.12 .mu.m per m.sup.3 while a normal room contains 530 million such particles per m.sup.3.
A photoresist is a formulation of organic compounds whose solubility changes when it is exposed to light radiation. Initially developed for the printing industry, the photoresist formulations have been used for about thirty years in microlithography for the fabrication of integrated circuits; their composition (weight %) is generally as follows:
resin =&gt; 25-55% photosensitizer =&gt; 2-30% solvents =&gt; 40-70% additives =&gt; 10-1000 ppm
Exposing a photoresist causes a chemical transformation which modifies its solubility in certain solvents or under certain pH conditions. Thus, the exposed regions of a positive photoresist are the seat of photochemical rearrangement reactions and become more soluble in alkaline solutions than those regions not exposed to the radiation. In the case of a Chemically Amplified Photoresist (CAP), the photoresist polymer is rendered soluble because, under illumination, a second chemical compound (acid photogenerator) releases an acid; this acid has a catalytic action on the polymer because it induces a chemical reaction causing the appearance of soluble groups in a basic developer. In the case of negative photoresist resins, the exposed parts are the seat of a photopolymerization reaction which results in the formation of a medium consisting of macromolecules whose molecular masses are much higher than those initially; it follows that there is an appreciable decrease in the solubility in an organic solvent.
Positive photoresists are generally easier to strip than negative photoresists. This is due to the fact that the positive resins to be stripped have not been exposed to the radiation, unlike the negative resins which have become modified and are often insoluble in most solvents. However, more than the PR itself, it is in fact a polymer called SWP (Side Wall Polymer) which forms on the sides of the patterns during plasma etching that is the most difficult to remove. Typically, this polymer contains metal atoms, carbon atoms and halogen (Cl, F, Br) atoms.
There are two methods for removing the photoresist from the wafer:
1 "dry stripping" or "ashing", which uses oxygen plasma, and PA1 2"wet stripping", which uses solvents.
These two methods do not act in the same way on the photoresist and are used, for most of the time, complementarily so as to combine the advantages of each of them. This is because dry stripping removes the PR better than the contaminants, and vice versa in the case of wet stripping. Dry stripping is particularly ineffective for removing SWPs.
Halogenated hydrocarbons (methylene chloride or tetrachloroethylene) were the first PR strippers used; they had the advantage of stripping both positive and negative PRs, with complete effectiveness. However, their high toxicity with respect to the environment and their excessively high volatility have forced the electronics industry to seek replacement formulations.